The myth that United complied with the regulations for overbooking through involuntary denial of boarding procedures is debunked by the lawyer who posted to Reddit. Obviously the attorney is familiar with aviation law.
This myth that passengers don't have rights needs to go away, ASAP. You are dead wrong when saying that United legally kicked him off the plane.
1. First of all, it's airline spin to call this an overbooking. The statutory provision granting them the ability to deny boarding is about "OVERSALES", specifically defines as booking more reserved confirmed seats than there are available. This is not what happened. They did not overbook the flight; they had a fully booked flight, and not only did everyone already have a reserved confirmed seat, they were all sitting in them. The law allowing them to denying boarding in the event of an oversale does not apply.
2. Even if it did apply, the law is unambiguously clear that airlines have to give preference to everyone with reserved confirmed seats when choosing to involuntarily deny boarding. They have to always choose the solution that will affect the least amount of reserved confirmed seats. This rule is straightforward, and United makes very clear in their own contract of carriage that employees of their own or of other carriers may be denied boarding without compensation because they do not have reserved confirmed seats. On its face, it's clear that what they did was illegal-- they gave preference to their employees over people who had reserved confirmed seats, in violation of 14 CFR 250.2a.
3. Furthermore, even if you try and twist this into a legal application of 250.2a and say that United had the right to deny him boarding in the event of an overbooking; they did NOT have the right to kick him off the plane. Their contract of carriage highlights there is a complete difference in rights after you've boarded and sat on the plane, and Rule 21 goes over the specific scenarios where you could get kicked off. NONE of them apply here. He did absolutely nothing wrong and shouldn't have been targeted. He's going to leave with a hefty settlement after this fiasco.